WASHINGTON — Challenging lawmakers, President Barack Obama will announce tonight in his State of the Union address that he’s raising the minimum wage for new federal contract workers to $10.10 an hour, underscoring a go-it-alone strategy in an election year critical to both parties and the balance of power in Congress. Obama’s speech to a joint session of Congress and millions more watching at home will be wrapped in a unifying theme: The federal government can play a key role in increasing opportunities for Americans who have been left behind, unable to benefit from a recovering economy. Yet the president will deliver a split message, pressing issues that will distinguish him and other Democrats from Republicans in the 2014 midterm elections. Illustrating his willingness to act on his own, the White House says Obama will announce that he will sign an executive order increasing the minimum wage from $7.25 to $10.10 for new federal contracts. The measure would not affect private companies’ existing contracts, except it would apply to renewals if other terms of an agreement changed. As a result, the order would benefit far fewer workers than the number foreseen by advocates of federal contract employees. Still, the issue dovetails with what will be Obama’s broader call for an increase in the broader national minimum wage to $10.10 and for future increases to be tied to inflation. Obama last year had called for an increase in the national minimum wage to $9.